1. Field of the Invention
This invention relates to a turbo blood pump that is extra corporeal or At implantable in a patient's body so as to conduct circulatory assistance or extra corporeal circulation of blood for a cardiopulmonary bypass operation, hemo purification, or other treatments.
2. Description of the Related Art
Conventionally, a roller blood pump for pumping out blood by compressing a tube was often used for extra corporeal circulation. However, this kind of roller pump needs a special pump segment tube to be fit thereto, and the lack of strength and durability of the tube causes problems. A further important problem is that the roller type blood pump is bulky and difficult to handle. Thus it is difficult to use, for example, in the sterile area of an operating field. Therefore, in recent years, turbo blood pumps such as centrifugal pumps and axial flow pumps, which can be used, for example, in the sterile area of an operating field, have been developed. See, for example, Japanese Granted Patent No. 1914715, Japanese Unexamined Patent Publication No. 4-2358, Japanese Unexamined Patent Publication No. 6-218043, Japanese Granted Utility Model No. 1792315, Japanese Unexamined Patent Publication No. 2-99800, Japanese Unexamined Patent Publication No. 7-75667, and Japanese Unexamined Patent Publication No. 7-178165.
In the centrifugal pump, liquid is fed into a casing inlet and flows substantially perpendicular to an impeller axis. In comparison, in an axial flow pump liquid is fed into a casing and flows in parallel to the axis. When developing these pumps, it is necessary to design the suitable rotation number in order to feed liquid efficiently or feed liquid so as to assure a stable state in which, for example, flow separation, collision, vortex, cavitation or the like is not caused. That is, a relatively low rotation number in a range of 1,000-4,000 rpm is desired to feed liquid efficiently by means of a normal left ventricular assistance centrifugal type pump (flow rate: 5L/min, pressure head: 100 mm Hg). Further, a relatively high rotation number in a range of 9,000-30,000 rpm is desired to feed liquid efficiently by means of an axial flow pump.
The discharge rate (flow rate) of the above turbo type pumps is determined mainly by the rotation speed and the size of the impeller. Thus, to assure a large discharge rate in a centrifugal type pump, the size of the pump including the impeller and casing needs to be enlarged. Generally, a centrifugal pump used for extra corporeal circulation has an impeller diameter as large as 40-80 mm and a large priming volume. However, as will be described later, enlargement of the pump is not desirable.
On the other hand, because the axial flow pump ensures a larger rotation number to achieve efficient fluid feeding than the centrifugal pump, it can provide a large discharge rate if the same impeller diameter is used. However, conventional axial flow pumps could not provide a sufficient driving power for cardiopulmonary bypass, because they were about 5-16 mm in impeller diameter. Furthermore, the axial flow pump has a higher impeller rotation number than the centrifugal pump, and, therefore, blood injury such as hemolysis is more likely to occur. Thus, usually, the axial flow pump is designed so that the rotation speed is low and the impeller vane is as large as possible, in order to reduce a possibility of hemolysis. However, in this case, hydraulic efficiency drops so that the advantage of the axial flow pump is not fully realized.
Further, because the conventional turbo blood pump of, for example, a centrifugal pump, is connected to a driving source, many types have shaft sealing. However, a blood pump having shaft sealing is difficult to operate for more than two weeks because a lack of durability of th-e shaft sealing. Also, because thrombus is likely to occur, shaft sealing is a problem with respect to anti-thrombogenicity. To solve the above problem, an impeller that does not require shaft seals was developed. Such an impeller is disclosed in U.S. Pat. No. 4,507,048 issued to Belenger et al. According to this invention, the upper and lower ends of the rotary shaft of the impeller are supported in the casing. External to the casing is a magnetic driving mechanism such as an electric coil which generates a rotating magnetic field for driving the impeller. A disadvantage of this invention is that rotor stability is lost when the supporting means is deformed. Deformation is caused by variations in the distance between the upper and lower bearings and the contacting pressure of the upper and lower supporting structure. Such deformation may cause hemolysis and thrombus formation in the blood pump.